Frequency hopping is a radio communication technique in spread-spectrum modulation wherein information is transmitted using a plurality of operating frequencies changing at set times to produce a narrow band signal that bounces or hops around in center frequency over the available spectrum.
In a centrally controlled multicellular mobile radio communication system based on slow frequency hopping, each cell has a base station that provides the necessary timing and control information received and used by all the remote stations that belong to the cell.
All stations belonging to a cell, i.e., the base station and all remote stations must hop in synchronism in order to communicate with each other at the same frequency. Different cells will typically operate on different frequency hopping patterns. The control information required to synchronize frequency hopping is broadcasted by the base station. A key problem in the operation of a frequency-hopping based system is that of a rapid acquisition of hop synchronization between one remote station and a base station.
The following references are typical of the background art in the field of frequency hopping systems and synchronization techniques thereof.
U.S. Pat. No. 5,081,641 discloses a method and apparatus for facilitating communication of information in a system without the use of a baseband hopping unit, by sharing a common TDM bus between a plurality of radio communication units, processing units, and information links, where the processing units extract traffic channel information, packetize and/or unpacketize the information, and return same back to the common bus for retrieval by the information links or radio communication units.
In U.S. Pat. No. 4,850,036 entitled, "Radio Communication System Using Synchronous Frequency Hopping Transmissions" a frequency-hopping radio communication system is disclosed comprising a control unit which transmits to and receives from each of a plurality of slave stations using a frequency-hopping mode of operation. During a start-up mode, the control unit communicates a starting message to each slave station using a predefined frequency. The message identifies to each slave station a frequency-hopping sequence to be used to select the frequencies from a group of frequencies for transmission to and reception from the control unit. This message also specifies to each slave station unique starting frequencies in the frequency-hopping sequence at which to begin transmitting and receiving. All slave station transmission are synchronized to the control unit transmissions, thereby preventing any two stations from concurrently using the same frequencies for either transmitting to or receiving from the control unit.
In U.S. Pat. No. 5,446,769 a method for handing off user devices between network control processors (NCPs) in a frequency hopping communication system includes establishing a set of synchronization frequencies for use by the NCPs, informing the plurality of user devices of the synchronization frequency and assigning a synchronization frequency to each respective NCP. Each NCP transmits synchronization at this assigned synchronization frequency during each hopping dwell. Upon a determination that a user device hand-off is necessary, this user device tunes to a synchronization frequency employed by the currently serving NCP and synchronizes its operation to the NCP transmitting synchronization information on the tuned to synchronization frequency.
While the solutions of the prior art are efficient in their environment, the duration for selecting a base station does not comply with the fast base selection duration required for the actual hand-off systems.
Moreover, the use of signalling channels for synchronization purposes as described, does not provide for compliance with various regulations of nations which may be required to control the duration of use of the available frequencies for signalling purposes according to the duration of use of the available frequencies for data transmission.